The growth in partisan polarization in US national politics over the past several decades has been mirrored at the state level, a development whose causes are still not fully understood. Although organized groups are theorized to pull parties to the ideological extremes, previous analyses of the relationship between interest group activity and polarization among state legislators have yielded mixed results. I explain these conflicting findings by drawing on the interest group literature to distinguish between two potential strategies groups can pursue—ideological (focused on electing legislators who share particular policy views) and access-oriented (prioritizing obtaining favors from incumbents)—and argue that the former approach, but not the latter, will lead to polarization. To determine the prevalence of these strategies, I implement a supervised machine learning model to classify PACs as ideological or access-oriented based on their donations to state legislative candidates, demonstrating that existing methods substantially underestimate the proportion of contributions that come from ideological sources. I further show that increased activity by ideological PACs in a state is associated with that state's legislature becoming more polarized. In addition, more extreme legislators rely on ideological PACs for a greater share of their funds, consistent with the idea that these groups promote polarization by supporting the election of polarized candidates. These results advance our understanding of interest group activity and the origins of partisan polarization in contemporary politics.